Add a fork-me banner (same as doc.crates.io)
Remove bogus about link for now

More links will probably be added later on once there's better integration
between the docs and the registry.

Closes #13
Rename Downloads => Install and link to it

The main page download button now links to the main install page

Closes #12
Add some more bottom links

"Install" just looked so lonely!
Mirror the style of the doc website for code font